Conventionally, there has been known a printing system comprised of an image forming apparatus, such as a printer, a facsimile machine, or a copying machine, and a post-processing apparatus, such as a finisher or a bookbinding machine. A post-processing apparatus performs post-processing, such as bookbinding, stapling, and so forth, on a sheet bundle output from an image forming apparatus. Post-processing apparatuses include an off-line post-processing apparatus, wherein a sheet bundle output from an image forming apparatus is temporarily stacked on a stacking device, such as a sheet discharge tray or a stacker, without being directly transferred to the post-processing apparatus, and then an operator brings the sheet material to the post-processing apparatus and causes the post-processing apparatus to perform processing.
On the other hand, a printing method called on-demand printing for performing printing of various kinds in small lots has been widely employed in image forming apparatuses, such as digital copying machines. This on-demand printing enables an image forming apparatus to meet the demand for printing of various kinds in small lots, and change print contents with ease. Therefore, the on-demand printing is suitable for printing product manuals, catalogs, booklets for individual users, prints for distribution in offices, and the like.
Further, there are various kinds of post-processing apparatuses each of which makes sheet bundles output from an associated image forming apparatus into booklets, such as catalogs or manuals. The post-processing apparatuses include, for example, a case binding machine that applies glue to a sheet bundle, bonds a cover sheet to the sheet bundle, and then performs cut-off processing on the same, a saddle stitching bookbinding machine that performs stapling in the center of a sheet bundle and then folds the same in two, and a ring binding machine that punches holes in a sheet bundle and binds the sheet bundle into a book using a special-purpose helical member.
By the way, when a comparison is made between a time period required for an image forming apparatus to complete printing and a time period required for a post-processing apparatus to complete post-processing so as to produce a booklet, the latter is generally longer than the former, though an exceptional case can occur depending on the number of pages of a booklet. This is because it is required to manually set sheet bundles one by one in the post-processing apparatus off-line and then start processing by the post-processing apparatus, though processing time taken by the off-line post-processing apparatus is shorter.
For this reason, in on-demand printing, as the number of booklets to be produced increases, in addition to time which it takes for the image forming apparatus to output sheet bundles, it takes a longer time for the post-processing apparatus to complete post-processing, and hence it takes a long time to obtain the booklets after the start of the printing.
Further, an operator, who handles the post-processing apparatus operating off-line and carries out other work while checking the operating state of the image forming apparatus, cannot always fetch a sheet bundle output from the image forming apparatus upon completion of the operation of the image forming apparatus. In such a case, the sheet bundle is left stacked on a stacking tray of the image forming apparatus. At this time, if there is no empty stacking tray, the image forming apparatus is not permitted to start a next print job, which results in an inefficient state of the apparatus.
To solve this problem, there have conventionally been disclosed the following techniques:
A first device configured to prevent generation of wait time due to interruption of a print job, by comparing the number of sheets that can be stacked on a sheet stacker with the number of sheets to be output in the print job and giving a warning when it is impossible to stack all the sheets to be output in the print job on the sheet stacker (see e.g. Japanese Patent Laid-Open Publication No. H09-240197); and
A second device configured to prevent generation of wait time due to interruption of a print job, by adding the number of sheets to be output in a subsequent print job to the number of sheets currently stacked on a sheet stacker, to thereby obtain a total value and then comparing the total value with an upper limit of the number of sheets that can be stacked on the sheet stacker, and inhibiting the start of the subsequent print job when the total value has exceeded the upper limit (see e.g. Japanese Patent Laid-Open Publication No. H10-029755).
In the above first and second devices, when interruption of a print job is predicted at the start of the print job, a warning that the print job will be interrupted is issued in advance, or the start of the print job is inhibited. Therefore, it is impossible to start post-processing to be performed on a sheet bundle immediately after completion of printout thereof from the image forming apparatus, and consequently a wait state occurs.